The EU has mildly outperformed the UK in overall economic growth by perhaps 1 percentage point over the last 5-6 years, i.e. ~7% vs ~6%. While of course both massively underperformed the USA. It's hard to avoid concluding that the actual effects of Brexit have been smaller than this kind of analysis suggests, and while we squabble about such things our countries are missing opportunity after opportunity.
Brexit harming the growth of both UK and EU is another perfectly valid interpretation of those numbers. I'm sure there's a little truth to both, and noise from all kinds of other factors.
[1] https://data.worldbank.org/indicator/NY.GDP.PCAP.CD?end=2024...